Like Monty Python's promise to television viewers 40 years ago, Michael Palin has long had an appetite for "something completely different". Writer, comedian, and modern day Phileas Fogg, the only constant in his career is change. Palin was at it again this week when the BBC announced his first small screen acting role for 22 years in first world war drama The Wipers Times, co-written by the Private Eye editor, Ian Hislop.

The former Python turned 70 in May, a week before he received the prestigious Bafta fellowship award which cemented his status as national treasure. But he retains a boyish enthusiasm that belies his age.

"No, no," he said after the Bafta ceremony when they asked if he had considered retirement. "I will die, but not retire."

For viewers of a certain age, Palin will always be the lumberjack with a penchant for women's clothing in Monty Python's Flying Circus, or the rhotacistic Pontius Pilate in Life of Brian.

Since the last Python film, 1983's The Meaning of Life, Palin has written and starred in The Missionary (and Terry Gilliam's Time Bandits and Brazil); won a Bafta for his role in John Cleese's A Fish Called Wanda; penned two novels; and had a brace of diaries published, with a third due next year.

But it is his TV travelogues for which he is best known today, a reinvention which began with BBC1's Around The World in 80 Days in 1989.

Inspired by the Jules Verne novel, Palin circumnavigated the world by train, ship, balloon and husky dog, returning to London just ahead of deadline only to be barred from filming in the Reform Club (where Fogg ended up in the novel).

Charmed by his everyman approach and the affection built up by Python, it was watched by more than 12 million viewers. Palin has been exploring the world ever since, embarking on six more epic journeys including from Pole to Pole, around the Pacific Rim, across the Himalayas and last year to Brazil.

It almost didn't happen; he was at least fourth choice for the job after Alan Whicker, Miles Kington, Noel Edmonds and (possibly) Clive James.

Palin said he was "blessed, or cursed ... with an insatiable curiosity, a desire to find something out about a people and a place". His first serious expedition was a 1972 US tour with Python cohort Terry Jones, with whom he wrote post-Python BBC2 comedy Ripping Yarns. It is Ripping Yarns – and in particular the opening episode, Tomkinson's Schooldays – that Palin is most proud (his favourite Python sketch is the fish-slapping dance, with Cleese).

"I'm slightly disappointed he is not writing comedy any more because he is such a funny man," says Jones of Palin.

The pair's six-state trip around the US encompassed New York and New Orleans before ending up on the west coast. "We went to Albuquerque, because it had a funny name, and Houston," says Jones. "Everyone said, 'Why are you coming to Houston?' We thought that was where the rockets set off from. It turns out it was just the control centre."

He adds that Palin "always wanted to explore the world and find out about people. What you see on screen is his nature."

Palin's travelogues, which officially began with a BBC Great Railway Journeys of the World in 1980 ("Points of View flatters me wonderfully," he wrote in his diary. "I really seem to have tapped the ageing, middle-class audience") also earned him a "Mr Nice" reputation he has not always enjoyed. "Nice means nothing," he once complained. "Is it someone who doesn't swear and shout? I swear and shout. Nice sounds ineffectual."

Born in Sheffield in 1943 and educated at Shrewsbury public school, Palin set on a career in comedy after taking part in an Oxford University revue show in Edinburgh in 1964. It was at Oxford that he met Jones, with whom he went to work on The Frost Report and Do Not Adjust Your Set, before they teamed up with Cleese, Gilliam, Graham Chapman and Eric Idle. Monty Python's Flying Circus was born on 5 October, 1969.

Palin puts the chances of a Python reunion – it could never be complete, after Chapman's death in 1989 – at "about 90 to one against … I would never say never to anything, whether it's a new series or learning the trombone. It might just happen. We get on well when we are together. We still make each other laugh a lot."

But it was Palin who vetoed the idea of a full-scale reunion following a one-off get together to mark Python's 30th anniversary in 1999.

"Mike didn't want to do it," says Jones. "I think he thought getting a load of old men on stage would have been a disappointment for our fans."

"That didn't make me very popular," Palin admits later. "I was worried about putting together a show that was less good than things we'd done before."

If there is one thing that unites Palin's work, it is a nagging self-doubt that affects him "every time, every day," he says, despite the affection and acclaim for a body of work that spans almost 50 years.

"I do have high standards. I look at everything I have done and think, why wasn't that better? Part of my motivation is from crippling self-doubt – I have got to prove myself wrong.", down the line from his home in Gospel Oak, north London.

Palin married his wife, Helen, in 1966, after a holiday romance in Southwold, Suffolk. They have three children and two grandchildren. Palin suffered a personal tragedy with the suicide of his older sister, Angela, in 1987. Long-standing family friend, and former chief executive of the Royal Television Society, Simon Albury, says: "Being a grandfather is the most rewarding thing in his [Palin's] life. He is very much a family man, an old friends' man. Some people as they rise up cast their friends aside, but he has never done that. He is surprisingly untouched by celebrity."

Palin once said that his long absences filming "can be very helpful in keeping a relationship alive … You work out a way of dealing with long absence – either that or the marriage ends."

But the stress was at its most intense when his wife had surgery for a benign brain tumour in 1997, with Palin filming 7,000 miles away in Borneo. She talked him out of coming home. He spoke to the surgeon on a satellite phone instead.

Palin has already filmed The Wipers Times, a drama about the satirical newspaper produced by British troops during the first world war, co-written by Hislop and cartoonist Nick Newman. His last TV acting role was the headmaster in Alan Bleasdale's acclaimed 1991 Channel 4 drama GBH.

Palin said he was attracted to the new project, which will co-star Ben Chaplin, Julian Rhind-Tutt and Emilia Fox, because of the writers and because he has "always been fascinated by all things to do with the first world war". (He previously presented a Timewatch documentary about the conflict's last day, on which 11,000 soldiers were killed.)

"It deals with an area which is rarely covered: the importance of humour in war," he says of The Wipers Times. "Very often when the most grim things are happening, the bitterest humour comes out. It is also a fascinating true story."

The filming came during a rare break from trips overseas, with Palin occupied by the third volume of his diaries, which will go up to 1998, and a documentary about the US realist painter Andrew Wyeth for BBC2.

The paperback edition of Palin's second novel, The Truth, is published in the next few weeks, and he is considering whether he should embark on a third.

But will there be an another expedition? "If I say no I would probably be lying because I have said no before. Quite honestly at the moment I just don't know." Last year's Brazil and 2007's New Europe did not have the impact (or perhaps the narrative journey) of his earlier trips.

But there is no shortage of ambition. "The Middle East is somewhere for fairly obvious reasons we haven't been able to work," says Palin. "I would love to go to Iran. The island of Madagascar, everyone says is pretty exotic, or the wonderful Namibian desert."

A former chairman of Transport 2000 (now the Campaign for Better Transport) and a passionate advocate of geography teaching in schools, Palin eschews talk of politics since he once suggested he was more of a John Prescott than Tony Blair man, a tag he found hard to discard. He once dismissed romantic notions of politics as "bullshit, absolute bullshit. It's an extremely practical business of how you retain power."

The closest Palin came to scandal (of a sort) was his 1988 role as stammering hitman Ken Pile in A Fish Called Wanda. (His father, a short-tempered man who never understood Monty Python, had an uncontrollable stammer).

Palin's performance was criticised by some health campaigners and, in response, he gave his name to Britain's first specialist centre for children with a stammer in north London.

He started a diary in his mid-20s and described the habit as a "tenacious parasite". To stop, he says, "would be like having a limb cut off".

Cleese nominated Palin as his luxury item on Desert Island Discs, although the then presenter, Sue Lawley, insisted he could only have him stuffed. "'Yap, yap yap', he goes, all day long and through the night," said Cleese. "And then, when everyone else has gone to bed, he writes a diary."

CV

1943 Born in Sheffield, educated at Shrewsbury School

1969 Monty Python's Flying Circus begins on BBC

1976 First series of Ripping Yarns

1982 Writes, stars in The Missionary

1983 Last Python film, Meaning of Life

1988 Bafta-winning role in A Fish Called Wanda

1989 Around The World in 80 Days

1991 Starred in Alan Bleasdale's GBH

1992 Pole to Pole

1995 Debut novel, Hemingway's Chair

2004 Himalaya

2006 First volume of diaries, The Python Years 2009 Second volume, Halfway to Hollywood